Development of new treatment strategies for demyelinating disorders such as multiple sclerosis (MS), as well as assessment of disease status and efficacy of existing therapies, has been constrained by the lack of reliable biochemical measures of disease activity. The fundamental processes underlying disease progression or resolution in MS are rates of brain myelin breakdown and synthesis. Biomarkers of flux rates through these biochemical pathways in vivo (i.e., kinetic biomarkers), particularly for the remyelination pathway, had not previously been available, however. We recently developed a technique for measuring the synthesis rate of brain myelin based on the incorporation of 2H (deuterium) into the galactose moiety of galactocerebroside (GalC) during oral intake of 2H2O (heavy water), using gas chromatographic-mass spectrometric (GC/MS) analyses. Studies in rodents showed highly reproducible myelin kinetics and sensitive detection of reduced and increased myelin synthesis rates during cuprizone intake and the postcuprizone recovery periods, respectively. Moreover, plasma GalC 2H-enrichments appeared to reflect brain GalC labeling in rats and could be measured by GC/MS from plasma of normal human subjects. Our specific aims in this phase I proposal are: (1) To validate the use of plasma GalC synthesis as a measure of brain myelin turnover in rodents under normal conditions and during demyelination and remyelination (cuprizone treatment) (2) To assess the adequacy of 10 ml blood draw for measurement of plasma GalC synthesis in humans; (3) To carry out pilot studies of GalC synthesis by the 2H2O technique in normal human subjects. In summary, we propose in this phase I project to develop and test a biomarker for directly measuring myelin synthesis rates that can be used in humans. A number of highly strategic applications of this tool can be envisioned for basic research, drug development, and patient management. In particular, these tools may allow development of new therapeutics aimed at stimulating remyelination. In phase II we plan to apply the plasma GalC kinetic biomarker in patients with MS to investigate aspects of disease activity and response to therapeutic interventions.